1. Field of the Invention
The present invention generally relates to medical methods and apparatus. More particularly, the present invention relates to methods and apparatus used to couple a prosthesis to a spinal segment and adjust the prosthesis during orthopedic internal fixation procedures. This includes but is not limited to treatment of patients having back pain or other spinal conditions.
A major source of chronic low back pain is discogenic pain, also known as internal disc disruption. Patients suffering from discogenic pain tend to be young, otherwise healthy individuals who present with pain localized to the back. Discogenic pain usually occurs at the discs located at the L4-L5 or L5-S1 junctions of the spine. Pain tends to be exacerbated when patients put their lumbar spines into flexion (i.e. by sitting or bending forward) and relieved when they put their lumbar spines into extension (i.e. by standing or arching backwards). Flexion and extension are known to change the mechanical loading pattern of a lumbar segment. When the segment is in extension, the axial loads borne by the segment are shared by the disc and facet joints (approximately 30% of the load is borne by the facet joints). In flexion, the segmental load is borne almost entirely by the disc. Furthermore, the nucleus shifts posteriorly, changing the loads on the posterior portion of the annulus (which is innervated), likely causing its fibers to be subject to tension and shear forces. Segmental flexion, then, increases both the loads borne by the disc and causes them to be borne in a more painful way. Discogenic pain can be quite disabling, and for some patients, can dramatically affect their ability to work and otherwise enjoy their lives.
Pain experienced by patients with discogenic low back pain can be thought of as flexion instability, and is related to flexion instability manifested in other conditions. The most prevalent of these is spondylolisthesis, a spinal condition in which abnormal segmental translation is exacerbated by segmental flexion. The methods and devices described herein should as such also be useful for these other spinal disorders or treatments associated with segmental flexion, for which the prevention or control of spinal segmental flexion is desired. Another application for which the methods and devices described herein may be used is in conjunction with a spinal fusion, in order to restrict motion, promote healing, and relieve pain post-operatively. Alternatively, the methods and devices described should also be useful in conjunction with other treatments of the anterior column of the spine, including kyphoplasty, total disc replacement, nucleus augmentation and annular repair.
Patients with discogenic pain accommodate their syndrome by avoiding positions such as sitting, which cause their painful segment to go into flexion, preferring positions such as standing, which maintain their painful segment in extension. One approach to reducing discogenic pain involves the use of a lumbar support pillow often seen attached to office chairs. Biomechanically, the attempted effect of the ubiquitous lumbar support pillow is also to maintain the painful lumbar segment in the less painful extension position.
Current treatment alternatives for patients diagnosed with chronic discogenic pain are quite limited. Many patients follow a conservative treatment path, such as physical therapy, massage, anti-inflammatory and analgesic medications, muscle relaxants, and epidural steroid injections, but typically continue to suffer with a significant degree of pain. Other patients elect to undergo spinal fusion surgery, which commonly requires discectomy (removal of the disk) together with fusion of adjacent vertebra. Fusion may or may not also include instrumentation of the affected spinal segment including, for example, pedicle screws and stabilization rods. Fusion is not lightly recommended for discogenic pain because it is irreversible, costly, associated with high morbidity, and has questionable effectiveness. Despite its drawbacks, however, spinal fusion for discogenic pain remains common due to the lack of viable alternatives.
An alternative method, that is not commonly used in practice, but has been approved for use by the United States Food and Drug Administration (FDA), is the application of bone cerclage devices which can encircle the spinous processes or other vertebral elements and thereby create a restraint to motion. Physicians typically apply a tension or elongation to the devices that applies a constant and high force on the anatomy, thereby fixing the segment in one position and allowing effectively no motion. The lack of motion allowed after the application of such devices is thought useful to improve the likelihood of fusion performed concomitantly; if the fusion does not take, these devices will fail through breakage of the device or of the spinous process to which the device is attached. These devices are designed for static applications and are not designed to allow for dynamic elastic resistance to flexion across a range of motion. The purpose of bone cerclage devices and other techniques described above is to almost completely restrict measurable motion of the vertebral segment of interest. This loss of motion at a given segment gives rise to abnormal loading and motion at adjacent segments, which can lead eventually to adjacent segment morbidity.
An alternative solution that avoids some of the challenges associated with cerclage devices involves the use of an elastic structure, such as tether structures, coupled to the spinal segment. The elastic structure can relieve pain by increasing passive resistance to flexion while often allowing substantially unrestricted spinal extension. This mimics the mechanical effect of postural accommodations that patients already use to provide relief.
Spinal implants using tether structures are currently commercially available. One such implant couples adjacent vertebrae via their pedicles. This implant includes spacers, tethers and pedicle screws. To install the implant, selected portions of the disc and vertebrae bone are removed. Implants are then placed to couple two adjacent pedicles on each side of the spine. The pedicle screws secure the implants in place. The tether is clamped to the pedicle screws with set-screws, and limits the extension/flexion movements of the vertebrae of interest. Because significant tissue is removed and because of screw placement into the pedicles, the implant and accompanying surgical methods are highly invasive and the implant is often irreversibly implanted. There is also an accompanying significant chance of nerve root damage. Additionally, the tip of the set-screw clamps the tethers, and this may result in abrasion of the tethers along with generation of particulate wear debris.
Other implants employing tether structures couple adjacent vertebrae via their processes instead. These implants include a tether and a spacer. To install the implant, the supraspinous ligament is temporarily lifted and displaced. The interspinous ligament between the two adjacent vertebrae of interest is then permanently removed and the spacer is inserted in the interspinous interspace. The tether is then wrapped around the processes of the two adjacent vertebrae, through adjacent interspinous ligaments, and then mechanically secured in place by the spacer or also by a separate component fastened to the spacer. The supraspinous ligament is then restored back to its original position. Such implants and accompanying surgical methods are not without disadvantages. These implants may subject the spinous processes to frequent, high loads during everyday activities, sometimes causing the spinous processes to break or erode. Furthermore, the spacer may put a patient into segmental kyphosis, potentially leading to long-term clinical problems associated with lack of sagittal balance. The process of securing the tethers is often a very complicated maneuver for a surgeon to perform, making the surgery much more invasive. And, as previously mentioned, the removal of the interspinous ligament is permanent. As such, the application of the device is not reversible.
More recently, less invasive spinal implants have been introduced. Like the aforementioned implant, these spinal implants are placed over one or more pairs of spinous processes and provide an elastic restraint to the spreading apart of the spinous processes occurring during flexion. However, extension-limiting spacers are not used and interspinous ligaments are not permanently removed. As such, these implants are less invasive and may be reversibly implanted. The implants typically include a tether structure and a securing mechanism for the tether. The tether may be made from a flexible polymeric textile such as woven polyester (PET) or polyethylene (e.g. ultra high molecular weight polyethylene, UHMWPE); multi-strand cable, or other flexible structure. The tether is wrapped around the processes of adjacent vertebrae and then secured by the securing mechanism. The securing mechanism may involve the indexing of the tether and the strap, e.g., the tether and the securing mechanism includes discrete interfaces such as teeth, hooks, loops, etc. which interlock the two. Highly forceful clamping may also be used to press and interlock the tether with the securing mechanism. Many known implementations clamp a tether with the tip of a set-screw, or the threaded portion of a fastener. However, the mechanical forces placed on the spinal implant are unevenly distributed towards the specific portions of the tether and the securing mechanism which interface with each other. These portions are therefore typically more susceptible to abrasion, wear, or other damage, thus reducing the reliability of these spinal implants as a whole. Other known methods use a screw or bolt to draw other components together to generate a clamping force. Other locking methods include the use of a friction fit and are disclosed in greater detail below. While these methods may avoid the potentially damaging loads, the mechanical complexity of the assembly may be increased by introducing more subcomponents.
A key to proper implantation of many of the spinous process constraint devices described above is adjusting the tension or size of the device when wrapped around the spinous processes. If the band is not properly adjusted, it may be too loose and therefore may disengage from the anatomy, or it may not provide adequate resistance to flexion resulting in failure to alleviate the pain or instability. On the other hand, if the band is too tight or too small, the device may provide too much resistance to flexion and unnecessarily restrict the spinal segment's ability to bend, or effect higher than necessary loads to portions of the vertebrae or soft tissues. It is therefore imperative to properly adjust the size and/or tension of the spinous process device. The device ideally should have a predetermined and preferred configuration while the patient is in a preferred posture (e.g. the standing position) and the device should also provide a force resistive to flexion of the spinal segment while still allowing significantly unrestricted extension of the spinal segment. For the aforementioned reasons, it would be desirable to provide improved methods and apparatus for coupling a prosthesis to a spinal segment and adjusting the prosthesis, especially during orthopedic internal fixation procedures. In particular, such methods and apparatuses should be easy to perform and be minimally invasive.
2. Description of the Background Art
Patents and published applications of interest include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US 2005/0049708; 2005/0192581; 2005/0216017; US 2006/0069447; US 2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096; 2008/0009866; 2008/0108993; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO2004/052246 A1; WO 2004/073532 A1; WO2008/051806; WO2008/051423; WO2008/051801; WO2008/051802; and Published Foreign Application Nos. EP0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexible constraints applied to spinal segments are described in Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al. (2002) Eur. Spine J. 5186-5191; Al Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol 2:259-264.